But a cocktail is forbidden (4)
I believe the answer is:
tabu
'is forbidden' is the definition.
(synonyms)
'but a cocktail' is the wordplay.
'cocktail' indicates anagramming the letters.
'but'+'a'='buta'
'buta' anagrammed gives 'TABU'.
(Other definitions for tabu that I've seen before include "Prohibited practice" , "beyond the pale" , "Socially prohibited" , "Forbidden (variant spelling)" , "Social prohibition" .)